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Simulation of the High Field BOLD Signal

Functional magnetic resonance imaging (fMRI) is an increasingly popular technique which allows the non-invasive measurement of brain activity. Most functional MR images rely on the blood oxygenation level dependent (BOLD) MR signal. The physiological origin of this signal, which depends on variables such as the cerebral blood volume (CBV), the cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2), is still not completely known and is an active field of research and debate. The aim of this project is the simulation of the BOLD MR signal using the general purpose MR simulator JEMRIS. Starting from given cerebral blood volume and cerebral blood oxygenation, a Monte Carlo approach is employed to generate a random microvascular structure as shown in fig. 1. As proposed by Ogawa et al. (1993), the blood vessels are represented by infinite cylinders. The individual spins then diffuse within the microvascular structure, experiencing the magnetic field distortions generated by the blood vessels while the MR sequence is solved using the numerical ordinary equation solver CVODE. This way the resulting BOLD signal can be investigated as a function of sequence parameters as well as depending on the physiological conditions (blood volume and blood oxygenation).

Simulation of the High Field BOLD SignalFig. 1: A typical randomly generated microvascular structure. Red: arterioles (r=8?m) Magenta: capillaries (r=2.5?m) Blue: venules (r=8?m)